Conductive contact elements and electric connectors

ABSTRACT

A conductive contact element is characterized by molding and curing a conductive silicone rubber composition comprising 
     (A) 100 parts by weight of an organopolysiloxane having at least two aliphatic unsaturated groups represented by the average compositional formula (1):
 
R 1   n SiO (4-n)/2   (1)
 
wherein R 1  is each independently a substituted or unsubstituted monovalent hydrocarbon group, and n is a positive number of 1.98 to 2.02,
 
     (B) 300–700 parts by weight of a granular silver powder having a tap density of up to 2.0 g/cm 3  and a specific surface area of up to 0.7 m 2 /g, and 
     (C) an effective amount to cure component (A) of a curing agent for component (A).

TECHNICAL FIELD

This invention relates to electroconductive contact elements and anelectric connector comprising the same, used for electric connection invarious electric and electronic equipment, typically in connecting asemiconductor package of the area array type like land grid array (LGA)or ball grid array (BGA) to a packaging board.

BACKGROUND ART

In the prior art, LGA or BGA semiconductor packages are connected topackaging boards by direct soldering or by means of movable pins heldfor vertical motion by leaf springs or coil springs. However, as thenumber of external connection terminals is increasing in currentsemiconductor packages tailored for higher performance and function,simultaneous connection using solder is deemed difficult from thestandpoint of connection reliability. Also in conjunction with thespeed-up of electric signals, the conventional leaf springs and coilsprings can now interfere with the high speed signal transmissionbecause they have an increased inductance component due to longdistances of connection.

In view of the above-discussed problems, electric connectors in which aplurality of conductive elastomer elements penetrate through and aresupported by an insulating substrate are recently under study. Siliconerubber compositions having metal powder compounded therein areadvantageously used as the material of which the electric connectors areconstructed. As the metal powder to be compounded in these siliconerubber compositions, silver powder is frequently used from thestandpoints of resistance and cost. The silver powder is classified intoa reduced silver powder obtained by reducing an aqueous silver nitratesolution with a reducing agent such as hydrazine, formaldehyde orascorbic acid, an electrolytic silver powder obtained throughelectrolysis of an aqueous silver nitrate solution for depositing silveron a cathode, and an atomized silver powder obtained by heat meltingsilver at or above 1,000° C. and atomizing the silver melt into water orinert gas. The shape of these silver powders is divided into granular,flaky, dendritic and irregular shapes. In general, the granular silverpowder has the tendency that particles agglomerate together, so thatcompounding of the silver powder in silicone rubber results in theinconsistency of resistance because the resistance value is likely tofluctuate depending on the dispersed state of silver particles. Thus,the granular silver powder is often used in combination with the flakysilver powder.

For the flaky silver powder, a certain manufacturing process is bygrinding a silver powder while treating it with saturated or unsaturatedhigher fatty acids such as lauric acid, myristic acid, palmitic acid,stearic acid, and oleic acid, metal soaps, higher aliphatic amines,polyethylene wax or the like. However, the flaky silver powder by such aprocess can retard the vulcanization of silicone rubber to which thepowder is added. It is also known that the silver powder which has notbeen treated as above results in the inconsistency of resistance ofsilicone rubber to which the powder is added.

An electric connector obtained by molding such a conductive siliconerubber composition in a mold or the like has an inconsistent conductionresistance when used in the mounting of a semiconductor package. As aresult, the operation of the semiconductor package becomes unstable.Further, if compression of the semiconductor package to the packagingboard is repeated, the agglomerated structure and chain of the silverpowder are disrupted, resulting in a drastic increase of conductionresistance to inhibit repeated use.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above-describedcircumstances, and its object is to provide conductive contact elementswhich have the consistency of conduction resistance when molded in amold and suppress and prevent the conduction resistance from increasingon repeated use, and an electric connector comprising the same.

Making extensive investigations to achieve the above object, theinventors have discovered that when conductive contact elements and anelectric connector are made using a silicone rubber composition havingblended therein a granular silver powder having a tap density of up to2.0 g/cm³ and a specific surface area of up to 0.7 m²/g as the silverpowder, it becomes possible to improve the dispersion and minimize theagglomeration of silver powder, to ensure the consistency of conductionresistance when molded in a mold, and to suppress or prevent theconduction resistance from increasing upon repeated use. The presentinvention is predicated on this discovery.

Accordingly, the present invention provides a conductive contact elementcharacterized in that a conductive silicone rubber compositioncomprising

(A) 100 parts by weight of an organopolysiloxane having at least twoaliphatic unsaturated groups represented by the average compositionalformula (1):R¹ _(n)SiO_((4-n)/2)  (1)wherein R¹ is each independently a substituted or unsubstitutedmonovalent hydrocarbon group, and n is a positive number of 1.98 to2.02,

(B) 300 to 700 parts by weight of a granular silver powder having a tapdensity of up to 2.0 g/cm³ and a specific surface area of up to 0.7m²/g, and

(C) an effective amount to cure component (A) of a curing agent forcomponent (A)

is molded and cured; and an electric connector comprising the conductivecontact elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of conductive contact elements and an electricconnector according to the present invention disposed between asubstrate and a semiconductor package.

FIG. 2 is a perspective view of the conductive contact elements andelectric connector according to the present invention.

FIG. 3 illustrates a method of preparing conductive contact elements andan electric connector according to one embodiment of the presentinvention; FIG. 3( a) being a cross-sectional view of a conductivesilicone rubber composition resting on a substrate which is set in amold; FIG. 3( b) being a cross-sectional view showing the heatcompression molded state with the mold of FIG. 3( a) clamped; FIG. 3( c)being a cross-sectional view showing an electric connector havingconductive contact elements integrally molded with the substrate, whentaken out of the mold.

FIG. 4 illustrates conductive contact elements and an electric connectoraccording to various embodiments of the present invention; FIG. 4( a)illustrating conductive contact elements of generally barrel shape incross section; FIG. 4( b) illustrating conductive contact elements ofgenerally thick column shape in cross section; FIG. 4( c) illustratingconductive contact elements of generally thin column shape in crosssection; FIG. 4( d) illustrating conductive contact elements ofgenerally track-field shape in cross section.

FIG. 5 illustrates conductive contact elements and an electric connectoraccording to alternative embodiments of the present invention; FIG. 5(a) illustrating conductive contact elements of generally circular shapein cross section; FIG. 5( b) illustrating conductive contact elements inwhich the edges of upper and lower end portions are rounded bychamfering; FIG. 5( c) illustrating conductive contact elements ofgenerally octagonal shape in cross section; FIG. 5( d) illustratingconductive contact elements having side walls which are partiallycurved.

THE BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described in further detail.

The electroconductive contact elements or pads of the invention arethose obtained by molding and curing the above-describedelectroconductive silicone rubber composition.

Component (A) of the conductive silicone rubber composition is anorganopolysiloxane having at least two aliphatic unsaturated groupsrepresented by the average compositional formula (1) shown above.

In formula (1), R¹ which may be the same or different is a substitutedor unsubstituted monovalent hydrocarbon group, preferably having 1 to 10carbon atoms, more preferably 1 to 8 carbon atoms, typically selectedfrom among alkyl groups such as methyl, ethyl, propyl and butyl,cycloalkyl groups such as cyclohexyl, alkenyl groups such as vinyl,allyl, butenyl and hexenyl, aryl groups such as phenyl and tolyl,aralkyl groups such as benzyl and phenylethyl, and the foregoing groupsin which some or all of the hydrogen atoms attached to carbon atoms aresubstituted with halogen atoms, cyano groups or the like, such aschloromethyl, trifluoropropyl and cyanoethyl. Preferably R¹ is a methyl,vinyl or phenyl group, and more preferably methyl accounts for at least50 mol %, especially at least 80 mol % of the entire R¹ groups.

The organopolysiloxane represented by the average compositional formula(1) should have at least two aliphatic unsaturated groups (especiallyalkenyl groups), while the content of aliphatic unsaturated groups in R¹is preferably in a range of 0.001 to 20 mol %, especially 0.025 to 5 mol%. It is noted that the aliphatic unsaturated groups may be positionedat the ends of a molecular chain or side chains on a molecular chain, orboth the ends and side chains.

The letter n is a positive number of 1.98 to 2.02. Generally theorganopolysiloxane represented by the average compositional formula (1)is preferably of straight chain although a mixture of one or moreorganopolysiloxanes differing in molecular structure or molecular weightis acceptable. The organopolysiloxane should preferably have an averagedegree of polymerization of 100 to 10,000, especially 3,000 to 20,000.

Component (B), a second essential component in the invention is agranular silver powder having a tap density of up to 2.0 g/cm³ and aspecific surface area of up to 0.7 m²/g.

In general, the constants representing the agglomeration of silverpowder include a tap density (ISO 3953–1977) and a BET specific surfacearea. The silver powder used herein should have a tap density of up to2.0 g/cm³ and a BET specific surface area of up to 0.7 m²/g. Their lowerlimits may be selected as appropriate although it is preferred that thetap density be at least 0.05 g/cm³, especially at least 0.1 g/cm³, andthe BET specific surface area be at least 0.05 m²/g, especially at least0.1 m²/g.

Such silver powder is commercially available as Silbest F20 (TokurikiChemical Research Co., Ltd.).

The particle size of the silver powder used herein is not criticalalthough a particle size in the range of 0.05 to 100 μm is preferred,with an average particle size in the range of 1 to 10 μm beingpreferred. To form a silicone rubber having a low resistance, it ispreferred that silver particles be partially joined rather than beingcompletely independently dispersed.

The process of preparing a silver powder raw material as used herein isnot particularly limited. For example, electrolytic, grinding, heattreatment, atomizing and reduction processes are included. Of these, thereduction process is preferred because a powder having both a low tapdensity and a low BET specific surface area is readily obtainable bycontrolling process parameters.

The silver powder may be ground to a range meeting the above-definednumerical ranges prior to use. The apparatus for grinding the silverpowder is not particularly limited. For example, well-known apparatussuch as a stamp mill, ball mill, vibration mill, hammer mill, rollingmill and mortar are included.

The preferred amount of component (B) or silver powder blended is 300 to700 parts by weight, especially 400 to 600 parts by weight, per 100parts by weight of component (A) or organopolysiloxane. Less than 300parts by weight of component (B) or silver powder is too small toprovide a consistent resistance whereas more than 700 parts by weightdetracts from the mechanical properties of conductive silicone rubber,leading to reduced elasticity and degraded compression set.

It is noted that another conductive material other than the silverpowder as component (B) may be added to the conductive silicone rubbercomposition of the invention as long as the object of the invention isnot impaired.

Such conductive materials include conductive carbon black, conductivezinc white, conductive titanium oxide, etc., alone or in admixture oftwo or more.

The conductive carbon black used herein may be selected from thosecustomarily used in conventional conductive rubber compositions, andexamples include acetylene black, conductive furnace black (CF),super-conductive furnace black (SCF), extra-conductive furnace black(XCF), conductive channel black (CC), and furnace black and channelblack which have been heat treated at elevated temperatures of about1,500° C. Specific examples include acetylene blacks sold under thetrade name of Denka Acetylene Black from Denki Kagaku Kogyo K. K. andShawnigan Acetylene Black from Shawnigan Chemical Co.; conductivefurnace blacks sold under the trade name of Continex CF from ContinentalCarbon and Vulcan C from Cabot Corp.; super-conductive furnace blackssold under the trade name of Continex SCF from Continental Carbon andVulcan SC from Cabot Corp.; extra-conductive furnace blacks sold underthe trade name of Asahi HS-500 from Asahi Carbon Co., Ltd. and VulcanXC-72 from Cabot Corp.; and conductive channel black sold under thetrade name of Corax L from Degussa AG. Ketjen Black EC and Ketjen BlackEC-600JD (Ketjen Black International) which belong to a class of furnaceblack are also useful. Of these, acetylene black is more conductivebecause of a low impurity content and a well developed secondarystructure, and thus especially suited for use herein. Also useful areKetjen Black EC and Ketjen Black EC-600JD which exhibit highconductivity even at low loadings due to their outstanding surface area.

An example of white conductive titanium oxide is ET-500W by IshiharaIndustry Co., Ltd. It has a basic composition of TiO₂.SnO₂ preferablydoped with Sb. It is noted that the amount of the other conductivematerial added is preferably 1 to 500 parts by weight, especially 2 to300 parts by weight per 100 parts by weight of component (A).

Component (C), a third essential component of the inventive compositionis a curing agent for component (A). The curing mechanism of the curingagent is not particularly limited as long as it helps the conductivesilicone rubber composition vulcanize and cure by utilizing radicalreaction, addition reaction or the like as often used for thevulcanization of conventional silicone rubber compositions. A variety ofprior art well-known curing agents are useful. Specifically, organicperoxides are used for the radical reaction, and combinations ofplatinum base catalysts with organohydrogenpolysiloxanes are used forthe addition reaction. Inter alia, organic peroxides are preferred. Itis noted that the amount of the curing agent blended is an effectiveamount to cure component (A) or organopolysiloxane, as used inconventional conductive silicone rubber compositions.

More specifically, organic peroxide curing agents include benzoylperoxide, 2,4-dichlorobenzoyl peroxide, o-methylbenzoyl peroxide,p-methylbenzoyl peroxide, 2,4-dicumyl peroxide,2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane, di-tert-butyl peroxide,and tert-butyl perbenzoate. The amount of organic peroxide blended ispreferably 0.1 to 5 parts by weight per 100 parts by weight of component(A) or organopolysiloxane.

As to the curing agent for addition reaction, any well-known platinumbase catalysts may be used. Examples include platinum element alone,platinum compounds, platinum composites, chloroplatinic acid, andcomplexes of chloroplatinic acid with alcohol compounds, aldehydecompounds, ether compounds and olefins. Preferably the platinum basecatalyst is used in such amounts as to give 1 to 2,000 ppm of platinumatoms based on the weight of component (A) or organopolysiloxane.

The counterpart, organohydrogenpolysiloxane is one having at least two,especially at least three silicon atom-bonded hydrogen atoms (SiHgroups) in a molecule, preferably represented by the following averagecompositional formula.R_(a)H_(b)SiO_((4-a-b)/2)Herein, R is a monovalent hydrocarbon group as defined above for R¹,preferably free of aliphatic unsaturation, “a” and “b” are positivenumbers satisfying 0≦a≦3, 0<b≦3, and 0<a+b≦3, preferably 0≦a≦2.2,0.002<b≦2, and 1.002≦a+b≦3.

While the organohydrogenpolysiloxane used herein has at least two,especially at least three SiH groups in a molecule, these groups may bepositioned at the ends of or midway the molecular chain or both. Theorganohydrogenpolysiloxane preferably has a viscosity at 25° C. of 0.5to 10,000 mm²/S (cSt), especially 1 to 300 mm²/S.

The organohydrogenpolysiloxane may be either straight-chain, branchedchain or cyclic, and preferably has a degree of polymerization of up to300. Illustrative examples include diorganopolysiloxanes end-blockedwith a dimethylhydrogensilyl group, copolymers consisting ofdimethylsiloxane units, methylhydrogensiloxane units and terminaltrimethylsiloxy units, low-viscosity fluids consisting ofdimethylhydrogensiloxane units (H(CH₃)₂SiO_(1/2) units) and SiO₂ units,1,3,5,7-tetrahydrogen-1,3,5,7-tetramethylcyclotetrasiloxane,1-propyl-3,5,7-trihydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, and1,5-dihydrogen-3,7-dihexyl-1,3,5,7-tetramethylcyclotetrasiloxane.

The organohydrogenpolysiloxane is preferably added as the curing agentin such amounts that 50 to 500 mol % of hydrogen atoms directly bondedto silicon atoms (SiH groups) are available based on the aliphaticunsaturated groups (alkenyl groups) in component (A),organopolysiloxane.

In the conductive silicone rubber composition of the invention,reinforcing silica fillers such as silica hydrogel (hydrated silicicacid) and silica aerogel (silicic anhydride, fumed silica), fillers suchas clay, calcium carbonate, diatomaceous earth and titanium dioxide,dispersants such as low-molecular-weight siloxane esters anddiphenylsilane diol, heat resistance improvers such as iron oxide,cerium oxide and iron octylate, various carbon-functional silanes forimproving adhesion or moldability, halogen compounds for imparting flameretardance or the like may be admixed if necessary and as long as theobjects of the invention are not impaired.

In another embodiment, silica fines or the like may be premixed with thesilver powder as component (B) for preventing the silver particles fromagglomerating. The silica fines to be premixed preferably have aspecific surface area of at least 50 m²/g, and especially 100 to 300m²/g. Silica fines having a specific surface area of less than 50 m²/gmay be less effective for preventing agglomeration. The silica finesinclude, for example, fumed silica and precipitated silica. Such silicaswhich are surface treated with chlorosilanes, hexamethyldisilazane,organopolysiloxanes or alkoxysilanes for hydrophobicization are alsouseful. The amount of silica blended may be 0 to 5 parts by weight,especially 0.5 to 2 parts by weight per 100 parts by weight of component(B).

To impart thermal conductivity, alumina, quartz flour or boron nitridepowder may be added.

The silicone rubber composition used herein can be prepared by uniformlymixing the above-described components in a rubber mill such as atwin-roll mil, Banbury mixer or dough mixer (kneader), optionallyfollowed by heat treatment.

The present invention is directed to conductive contact elementsobtained by curing the above-described conductive silicone rubbercomposition and an electric connector comprising the conductive contactelements. When a semiconductor package is connected to a packagingboard, the conductive contact elements should preferably have a volumeresistivity of up to 1×⁻⁵ Ωm, especially up to 6×10⁻⁶ Ωm, in order tomaintain the conduction resistance at or below 50 Ωm.

The conductive contact elements and the electric connector are used forelectrical connection in various electric and electronic equipment,business machines, mobile phones, and information terminal units.Specifically, they are used for electrical connection in packagingboards (e.g., printed circuit boards, flexible printed circuit boards),semiconductor packages, liquid crystal displays, batteries, electricacoustic parts, and miniature electronic parts, which constitute suchequipment. Further, when conductive contact elements are supported by asubstrate, they are for the most part of columnar or frustoconicalshape. When conductive contact elements are not supported by asubstrate, they may be formed to generally linear, tape, rod or blockshape. The conductive contact elements may be either singular or plural.

Now referring to the drawings, a preferred embodiment of the inventionis described. As shown in FIGS. 1 and 2, an electric connector accordingto the preferred embodiment includes a flat packaging board 1 and asemiconductor package 10 of the LGA type serving as opposed first andsecond members to be electrically joined, an insulating substrate 20interposed between them and provided with a plurality of through-holes21, and a plurality of elastic conductive contact elements or pads 22fitted in and held by the through-holes 21 so that their opposite endportions protrude from the front and back sides of the insulatingsubstrate 20 for electrically connecting a plurality of electrodes 2 and11 on the packaging board 1 and the semiconductor package 10. Each ofthe elastic conductive contact elements 22 is formed of a cured productof the above-described conductive silicone rubber composition 23.

As shown in FIGS. 1 and 2, the insulating substrate 20 is formed as athin plate of substantially square shape as viewed in plane, using aglass-reinforced epoxy resin or any well-known engineering plastics(e.g., PET, PEN, PEI, PPS, PEEK, liquid crystal polymers, polyimides,etc.) and perforated with a plurality of circular, small diameter,through-holes 21 extending throughout the plate in a thicknessdirection. The preferred material of the substrate 20 is an engineeringplastic because of heat resistance, and especially a polyimide becauseof a low coefficient of thermal expansion. The thickness of thesubstrate 20 is preferably 25 μm to 3 mm, especially 50 to 200 μm forstrength and efficient manipulation. Preferably the through-holes 21 arearrayed at a pitch of 0.5 to 1.27 mm and perforated to a diameter of0.25 to 0.8 mm.

The conductive contact elements 22 each have a generally barrel shape incross section obtained by combining a pair of frustocones and are fittedin and held by the through-holes 21 in the insulating substrate 20 sothat their opposite end portions protrude from the front and back sidesof the insulating substrate 20 for electrically connecting theelectrodes 2 and 11 on the packaging board 1 and the semiconductorpackage 10.

As shown in FIG. 3, the conductive contact elements 22 are manufacturedby placing the conductive silicone rubber composition 23 in a mold 30and molding the composition therein. The mold 30 includes a pair ofupper and lower sections 30-a and 30-b having a plurality offrustoconical cavities 31 corresponding to the shape of the conductivecontact elements 22.

The method of manufacturing an electric connector using the mold 30 isdescribed. First of all, the substrate 20 is perforated with a pluralityof through-holes 21 in a thickness direction and in a matrix pattern bysuch a technique as laser drilling or etching, and in register with theelectrode 11 on the semiconductor package 10. Then the lower section30-a of the mold 30 is placed in contact with the back side of thesubstrate 20. The conductive silicone rubber composition 23 in an amountrequisite and sufficient to mold the conductive contact elements 22 isplaced on the surface of the substrate 20 so as to cover thethrough-holes 21. The upper section 30-b of the mold 30 is rested on theconductive silicone rubber composition 23 whereby the substrate 20 isvertically sandwiched in the mold 30 (between the upper and lower moldsections 30-a and 30-b) as seen from FIG. 3 a. With the mold 30 clamped,heat compression molding is carried out as seen from FIG. 3 b. Then theconductive silicone rubber composition 23 flows through and fills in thethrough-holes 21 in the substrate 20, and the conductive silicone rubbercomposition 23 is molded and cured into the substantially barrel shapein cross section. An electrical connector having the conductive contactelements 22 integrated with the substrate 20 is manufactured as seenfrom FIG. 3 c.

As shown in FIG. 1, the opposite end portions of the conductive contactelements 22 that protrude from the front and back sides of the substrate20 in the electric connector thus manufactured are placed in contactwith the electrodes 2 and 11 on the packaging board 1 and thesemiconductor package 10. With the electric connector clamped betweenthe packaging board 1 and the semiconductor package 10, the assembly iscompressed at a reduction of 20%. Then the packaging board 1 and thesemiconductor package 10 are kept in electric conduction through theelectric connector.

It is noted that since the conductive contact elements 22 is compressedbetween the packaging board 1 and the semiconductor package 10, therubber hardness is preferably in a range of 50 to 80, more preferably 60to 80 in durometer type A hardness according to JIS K6253. If the rubberhardness of the conductive contact elements 22 is less than 50, there isa likelihood that no sufficient repulsive load be exerted and stableconnection be unexpectable. With a rubber hardness of more than 80,there is a likelihood that the load required for compression becomelarge enough to damage the packaging board 1 and the semiconductorpackage 10.

In the above manufacturing method, since the conductive contact elements22 are molded from the conductive silicone rubber composition 23, it isunlikely that the conduction resistance changes largely or becomesunstable when molded in the mold 30. This stabilizes the operation ofthe semiconductor package 10 after mounting and enables continuouslong-term use of the semiconductor package 10. Even when compression ofthe semiconductor package 10 against the packaging board 1 is repeated,it is possible to fully accommodate signal transmission at a higherspeed because the conduction resistance is as low as 50 mΩ or less andthe influence of external noise is minimized due to a short distance ofconnection.

It is noted that although the conductive contact elements 22 are moldedto a generally barrel cross-sectional shape having a pair of frustoconescombined (as shown in FIG. 4 a) in the foregoing embodiment, the shapeof elements is not limited thereto and may be selected as appropriatedepending on the shape of electrodes on the packaging board 1 and thesemiconductor package 10, the load during conductive connection, andother factors. For example, the conductive contact elements 22 may beformed to a thick, generally column (either cylinder or prism)cross-sectional shape as shown in FIG. 4 b; a thin, generally columncross-sectional shape as shown in FIG. 4 d; or a generally track-fieldcross-sectional shape as shown in FIG. 4 d. Alternatively, theconductive contact elements 22 may be formed to a generally circular orelliptic cross-sectional shape as shown in FIG. 5 a; or the edges ofupper and lower flat end portions of the conductive contact elements 22may be rounded as shown in FIG. 5 b. It is also possible that theconductive contact elements 22 be formed to a generally octagonalcross-sectional shape as shown in FIG. 5 c; or the side walls of theconductive contact elements 22 be partially curved as shown in FIG. 5 d.The upper and lower shapes may be the same or different (asymmetric).From the standpoints of resistance and load, a generally barrelcross-sectional shape having a pair of frustocones combined as shown inFIG. 4 a is especially preferred.

EXAMPLE

Examples and Comparative Examples are given below for furtherillustrating the invention although the invention is not limitedthereto. In Examples, all parts are by weight.

Example 1

A conductive silicone rubber composition was prepared by blending 100parts of a methylvinylpolysiloxane consisting of 99.85 mol % ofdimethylsiloxane units and 0.15 mol % of methylvinylsiloxane units,end-capped with a dimethylvinylsilyl group, and having an average degreeof polymerization of about 8,000, 450 parts of granular silver powder A(average particle size 7.3 μm, tap density 1.4 g/cm³, specific surfacearea 0.6 m²/g, Silbest F20 by Tokuriki Chemical Research Co., Ltd.), and0.5 part per 100 parts of the mixture of the methylvinylpolysiloxane andthe silver powder A of dicumyl peroxide.

Next, a square insulating substrate of polyimide (100 μm thick) wasperpendicularly drilled at a pitch of 1 mm to form 1156 (34×34)through-holes having a diameter of 0.5 mm. As shown in FIG. 3, theconductive silicone rubber composition prepared above was placed on thesubstrate so as to cover a plurality of through-holes. The insulatingsubstrate at its front and back sides was clamped between the upper andlower sections of a mold where heat compression molding was carried outunder molding conditions of 160° C. and 5 minutes, manufacturing anelectric connector in which a plurality of conductive contact elementswere integrated with the substrate. Each conductive contact element hada generally barrel cross-sectional shape having a pair of frustoconescombined. For each conductive contact element, the generally barrelcross-sectional shape had a height of 1 mm, the narrow top of thefrustocone had a diameter of 0.5 mm, the dilated bottom of thefrustocone had a diameter of 0.6 mm, and the frustocone protruded 0.45mm from the front or back side of the substrate.

The electric connector thus manufactured is clamped between a packagingboard and a semiconductor package of the LGA type and compressed at areduction of 20% to bring the packaging board and the semiconductorpackage in electric conduction through the electric connector.Measurement of conduction resistance of all the conductive contactelements gave an average value of 17 mΩ and a maximum value of 30 mΩ,indicating that all the conductive contact elements had a low resistanceof up to 50 mΩ with a reduced variation. When compression operation wasrepeated 100 times, the resistance was found to have changed only 1.3times the initial value.

Based on these measurements, the conduction resistance was converted toa volume resistivity according to the following equation (2), providedthat the conductive contact element had a conduction cross-sectionalarea diameter of 0.5 mm and a conduction distance of 1 mm. For example,if the measurement of a conductive contact element is 50 mΩ, then thevolume resistivity is calculated to be 9.8×10⁻⁶ Ωm according to theequation.ρ=R×(A/L)  (2)Herein, ρ is a volume resistivity, R is a measurement of conductionresistance, A is a conduction cross-sectional area, and L is aconduction distance.

Example 2

Conductive contact elements and an electric connector were prepared asin Example 1 except that a conductive silicone rubber composition wasprepared by blending 100 parts of the methylvinylpolysiloxane, 500 partsof granular silver powder A (average particle size 7.3 μm, tap density1.4 g/cm³, specific surface area 0.6 m²/g, Silbest F20 by TokurikiChemical Research Co., Ltd.), and 0.5 part per 100 parts of the mixtureof the methylvinylpolysiloxane and the silver powder A of dicumylperoxide.

A similar test to Example 1 showed an average value of 10 mΩ and amaximum value of 15 mΩ, indicating a low resistance with a minimizedvariation. When compression operation was repeated 100 times, theresistance was found to have changed only 1.1 times the initial value.

Comparative Example 1

Conductive contact elements and an electric connector were prepared asin Example 1 except that a conductive silicone rubber composition wasprepared by blending 100 parts of the methylvinylpolysiloxane, 500 partsof a granular silver powder B (tap density 1.7 g/cm³, specific surfacearea 1.5 m²/g, AgC-Bo by Fukuda Metal Foil/Powder Industry Co., Ltd.),and 0.5 part per 100 parts of the mixture of the methylvinylpolysiloxaneand the silver powder B of dicumyl peroxide.

A similar test to Example 1 showed an average value of 42 mΩ and amaximum value of 120 mΩ, indicating a high initial conduction resistancewith a substantial variation. When compression operation was repeated100 times, the conduction resistance increased considerably, with nosatisfactory results being obtained.

Comparative Example 2

Conductive contact elements and an electric connector were prepared asin Example 1 except that a conductive silicone rubber composition wasprepared by blending 100 parts of the methylvinylpolysiloxane, 500 partsof a granular silver powder C (tap density 3.0 g/cm³, specific surfacearea 1.7 m²/g, AgC-D by Fukuda Metal Foil/Powder Industry Co., Ltd.),and 0.5 part per 100 parts of the mixture of the methylvinylpolysiloxaneand the silver powder C of dicumyl peroxide.

A similar test to Example 1 showed an average value of 51 mΩ and amaximum value of 220 mΩ, indicating a high initial conduction resistancewith a substantial variation. When compression operation was repeated100 times, some elements became non-conductive, with no satisfactoryresults being obtained.

TABLE 1 Specific Tap surface Comparative density area Example ExampleComposition (pbw) (g/cm³) (m²/g) 1 2 1 2 Methylvinylpolysiloxane 100 100100 100 Silver powder A 1.4 0.6 450 500 — — Silver powder B 1.7 1.5 — —500 — Silver powder C 3.0 1.7 — — — 500 Conduction Average  17  10  42 51 resistance (mΩ) Maximum  30  15 120 220 (mΩ) Minimum  9  6  19  23(mΩ) Volume resistivity 3.3 × 2.0 × 8.2 × 1.0 × (average, Ωm) 10⁻⁶ 10⁻⁶10⁻⁶ 10⁻⁵ Conduction resistance  22  11 218 660 after 100 repeatedcompressions (average, mΩ)

According to the invention, the conductive silicone rubber compositioncan be molded into conductive contact elements while the conductionresistance is kept stabilized. There are obtained the conductive contactelements that can prevent their conduction resistance from increasing onrepeated use as well as an electric connector comprising the same.

1. A conductive contact element characterized in that a conductivesilicone rubber composition comprising (A) 100 parts by weight of anorganopolysiloxane having at least two aliphatic unsaturated groupsrepresented by the average compositional formula (1):R¹ _(n)SiO_((4-n)/2)  (1) wherein R¹ is each independently a substitutedor unsubstituted monovalent hydrocarbon group, and n is a positivenumber of 1.98 to 2.02, (B) 300 to 700 parts by weight of a granularsilver powder having a tap density of up to 2.0 g/cm³ and a specificsurface area of up to 0.7 m²/g, and (C) an effective amount to curecomponent (A) of a curing agent for component (A) is molded and cured.2. The conductive contact element of claim 1 having a volume resistivityof up to 1×10⁻⁵ Ωm.
 3. An electric connector for providing electricconduction between opposed first and second members to be electricallyjoined, characterized by comprising an insulating substrate interposedbetween said opposed first and second members to be electrically joined,the insulating substrate being provided with a plurality ofthrough-holes, and conductive contact elements as set forth in claim 1or 2 fitted in and held by the through-holes such that opposite endportions thereof protrude from front and back sides of the insulatingsubstrate and contact said opposed first and second members to beelectrically joined.